It has become common to produce an article of manufacture through a process or system which is typically referred to as computer aided design and computer aided manufacturing (CAD/CAM). Through the use of computer systems programmed with CAD/CAM software, the time necessary to engineer, prototype and manufacture an article of manufacture has been substantially reduced.
The CAD software allows the designer to visualize a dimensionally scaled image of the article to be manufactured on a video display terminal of the computer which contains the CAD software. Through use of the CAD software, the designer can then engineer the article by acting only on the displayed video image of the article. With each iteration in the design sequence, the CAD software will calculate useful information regarding the article and graphically display such information. This information may include, inter alia, geometries of the article, such as size, shape, dimensions and points of intersection, and relevant textual information concerning the article. The information is generally displayable in conjunction with the video representation.
By reviewing the video image of the article and any displayed information, the designer can, over one or more iterations, interactively modify the video image and review the modifications in conjunction with the updated information. The final iteration results in an enhanced and perfected design of the article. The CAD software can then develop digital data which defines or describes the final design and from which the article can be manufactured. The data may be stored in any appropriate medium.
The data is interpreted by the CAM software and a machine tool operator to program a machine tool numerical controller with a set of (x,y,z) data points and the action to occur between each data point, such as the cutting tool path and the tool selection for example, to develop a machining sequence. The (x,y,z) data is usually referenced from a frame of reference origin within a machine table fixture which holds the machinable stock from which the article is fabricated during the machining sequence.
The fixture in turn is mounted to the machine tool table which has its own frame of reference origin. In the prior art, considerable time and effort is expended in the set up or design of the fixture and its proper location upon the machine tool table to obtain the correct offset between the fixture and table frame of reference origins. Following is an exemplary description of a typical prior art fixture set up procedure.
The set up man and numerical controller programmer first need to discuss how the article is to be held for machining, the location of the fixture frame of reference origin and part orientation, and how the programmer will program the machining sequence so as not to damage the fixture. The set up man then calculates the location of alignment pins, clamps and hold down bolts from the fixture reference frame origin. The subplate stock for the fixture must be cut and placed on the machine table. The machinist needs to accurately align the fixture subplate on the table by using a precision indicator and soft hammer. Once the subplate is properly located it is clamped into place and the numerical controller programmed with the hole locations for the dowel pins and other holding devices. The machine tool reference frame origin must be referenced very accurately to the fixture reference frame origin so that the hole locations in the subplate are machined in the calculated positions. However, this step is accomplished by manually jogging the machine tool from the table reference origin and visually placing it at the fixture reference origin and setting this offset between each reference origin into the controller. After the subplate is machined and the pins, clamps and stock are affixed thereto, the numerical controller may then be programmed with the machining sequence program for the article.
Therefore, a significant disadvantage and limitation of the prior art fixture setup procedure exists in the need to manually align the fixture on the machine tool table and further manually referencing the fixture reference origin to the machine tool table reference origin. Of course, the offset between the reference origins can only be calculated as precisely as the machinist "eyeballing" the fixture reference origin when placing the machine tool over it and the scale of the measuring instrument used to measure the distance between each reference origin.
Various prior art methods and apparatus have been developed for machine tool table fixture set up. The known prior art is as follows: U.S. Pat. No. 4,645,391 for Work Set Up Method and Apparatus; U.S. Pat. No. 4,168,464 for Set Point Sensing System for Numerically Controlled Machine Tools; U.S. Pat. No. 4,073,215 for Subplate Assembly for Machine Tools; and U.S. Pat. No. 3,867,763 for Setting Fixture for Machine Tools.